Modern motorized vehicles frequently include a powertrain that has an engine and an automatic transmission. Most automatic transmissions include a number of gear elements, generally in the nature of one or more planetary gear sets, for coupling the transmission's input and output shafts. Traditionally, automatic transmissions also include a number of hydraulically actuated torque transmitting devices, such as clutches and brakes, configured to selectively activate the aforementioned gear elements for establishing desired forward and reverse speed-ratios between the transmission's input and output shafts. A typical automatic transmission also includes a hydraulic circuit with a pump and solenoid valves to control the application of fluid pressure to each of the torque transmitting devices. The amount of torque transmitted through a typical automatic transmission is therefore dependent on the torque capacity of its torque transmitting devices, which is in turn proportional to fluid line pressure developed in the hydraulic circuit.
A transmission speed-ratio is generally defined as the transmission input speed divided by the transmission output speed. The transmission input shaft is generally selectively connectable to the vehicle engine, e.g., through a fluid coupling device such as a torque converter, whereas the output shaft is connected through a “drive train” directly to the vehicle wheels. Shifting from one speed-ratio to another is performed in response to engine throttle and vehicle speed, and generally involves releasing one or more “off-going” clutches associated with the current or attained speed-ratio, and applying one or more “on-coming” clutches associated with the desired or commanded speed-ratio.
To perform an “upshift”, the transmission transitions from a high speed-ratio to a low speed-ratio, while, conversely, to perform a “downshift”, the transmission transitions from a low speed-ratio to a high speed-ratio. An upshift or a downshift may be accomplished by disengaging a clutch associated with the current speed-ratio and contemporaneously engaging a clutch associated with the desired speed-ratio, thereby reconfiguring the gear set to operate at the desired speed-ratio. In order to achieve high quality shifting, shifts performed in the above manner require precise coordination between achieving desired torque capacity in the on-coming clutch, the engagement of the on-coming clutch, and disengagement of the off-going clutch.